Mistake in "Verb: Compound Past"
In the section Verbs: Compound past, there is a mistake. In lesson 1 they write "Je les ai mangés" and "Je les ai mangées", but this is wrong, as far as I know. It should be "Je les ai mangé". The last verb of the compound past only changes if the verb "être" is used. Then there can be a suffix, which can be -e, -s or -es depending on if it's female or plural form. So for exemple "Elles sont arrivées" or "Ils sont arrivés".
You are wrong, but it is not a disaster, you are in the process of learning, you are learning the French conjugation little by little. Try to read the link i found for you.
As Pierre says, "ce n'est pas grave". Learning means making errors and being corrected to learn some more. No-one ever stops learning!
In this case, the direct object "les" precedes the compound avoir verb therefore it needs to agree in gender and number with the direct object.
J'ai mangé les pommes. Je les ai mangées.
"I ate the apples." "I ate them"
Another good reference is :